Re: [HACKERS] WAL logging problem in 9.4.3?

On Tue, May 28, 2019 at 4:33 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
>
> On Mon, May 27, 2019 at 02:08:26PM +0900, Kyotaro HORIGUCHI wrote:
> > At Fri, 24 May 2019 19:33:32 -0700, Noah Misch <noah(at)leadboat(dot)com> wrote in <20190525023332(dot)GE1624191(at)rfd(dot)leadboat(dot)com>
> > > On Mon, May 20, 2019 at 03:54:30PM +0900, Kyotaro HORIGUCHI wrote:
> > > > Following this direction, the attached PoC works *at least for*
> > > > the wal_optimization TAP tests, but doing pending flush not in
> > > > smgr but in relcache.
> > >
> > > This task, syncing files created in the current transaction, is not the kind
> > > of task normally assigned to a cache. We already have a module, storage.c,
> > > that maintains state about files created in the current transaction. Why did
> > > you use relcache instead of storage.c?
> >
> > The reason was at-commit sync needs buffer flush beforehand. But
> > FlushRelationBufferWithoutRelCache() in v11 can do
> > that. storage.c is reasonable as the place.
>
> Okay. I do want this to work in 9.5 and later, but I'm not aware of a reason
> relcache.c would be a better code location in older branches. Unless you
> think of a reason to prefer relcache.c, please use storage.c.
>
> > > On Tue, May 21, 2019 at 09:29:48PM +0900, Kyotaro HORIGUCHI wrote:
> > > > This is a tidier version of the patch.
> > >
> > > > - Move the substantial work to table/index AMs.
> > > >
> > > > Each AM can decide whether to support WAL skip or not.
> > > > Currently heap and nbtree support it.
> > >
> > > Why would an AM find it important to disable WAL skip?
> >
> > The reason is currently it's AM's responsibility to decide
> > whether to skip WAL or not.
>
> I see. Skipping the sync would be a mere optimization; no AM would require it
> for correctness. An AM might want RelationNeedsWAL() to keep returning true
> despite the sync happening, perhaps because it persists data somewhere other
> than the forks of pg_class.relfilenode. Since the index and table APIs
> already assume one relfilenode captures all persistent data, I'm not seeing a
> use case for an AM overriding this behavior. Let's take away the AM's
> responsibility for this decision, making the system simpler. A future patch
> could let AM code decide, if someone find a real-world use case for
> AM-specific logic around when to skip WAL.
>

It seems there is some feedback for this patch and the CF is going to
start in 2 days. Are you planning to work on this patch for next CF,
if not then it is better to bump this? It is not a good idea to see
the patch in "waiting on author" in the beginning of CF unless the
author is actively working on the patch and is going to produce a
version in next few days.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com